The use of the L5 radio frequency band for the global radio locating system or GPS (the anglicized version of the “Navstar Global Positioning System”) in the civil sector or, of the E5 band for the European GALILEO system poses the problem of compatibility with the signals transmitted by DME.
Specifically, one of the problems posed by the transmission of data by spread spectrum radio signals, such as those of GPS signals or those of GALILEO, is the sensitivity of the receivers to jammers having regard to the low powers involved and to the considerable distances separating the transmitters and the receivers.
The signal in the frequency bands for civil GPS III is obtained by the modulation of a carrier by a first signal, “pilot pathway”, producing a modulation spectrum of a width of the order of 20 MHz and by a second quadrature signal modulated by the navigation message, “data pathway”, producing a second 20 MHz modulation spectrum.
FIG. 1 represents an exemplary pulsed signal transmitted by distance measuring equipment (DME) between a ground beacon and an aircraft. We shall subsequently refer to this signal as the DME signal. FIGS. 2a and 2b respectively represent the frequency profile of a pulse and of a pair of pulses.
The DME signal comprises two pulses spaced apart by a mode dependent predetermined time t0 (ARINC standard 709 A). For example, in the X mode of ground beacons, this spacing time t0 between the pulses is of the order of 12 μS. The pair of pulses is repeated with a frequency of the order of 2700 pulse pairs per second. Each pulse has a Gaussian shaped envelope and a beacon dependent carrier frequency.
Each ground beacon is recognized by the aircraft interrogating the beacon via the frequency of the pulse transmission channel. A database in the aircraft makes it possible to obtain the position and the frequency of each beacon. The transmission channels of the DME for the various beacons are 1 MHz apart.
Owing to its considerable coverage, the GPS receiver can receive in its frequency band a multitude of DME pulsed signals originating from several beacons situated on the ground. These signals transmitted at random instants and at frequencies spaced 1 MHz apart, cause degradation of the signal-to-noise ratio of the GPS receiver and, consequently, measurement errors.
In the general case, the effects of the interference of the pulsed signals on the receivers depend on numerous factors among which may be mentioned the peak power, the duty ratio, the pulse width.
GPS receivers in particular work with very low useful signal of levels of the order of −130 dBm, the level of the noise floor of the receiver. The type of coded digital modulation used in GPS receivers allows extraction of the useful signal for these weak reception levels. Under these conditions of very weak useful signal power received, a pulse of weak level at the input of the GPS receiver, of the order of 10 dB above the noise floor, may produce the saturation of the radiofrequency input stages and the saturation of the analog/digital converter of the receiver. Analog solutions may be developed to avoid these saturations. To be able to develop a digital solution, the radiofrequency input stages and the analog digital converter must be dimensioned so as to avoid this saturation while taking into account the interference environment.
To avoid disturbances of this type by DME a simple solution would consist in reassigning the frequencies of the DME, but if this solution is possible in the United States of America, it is not possible in Europe or in Japan on account of the high density of radio beacons.